1. Field of the Invention
The present invention relates to a control change system for a hydraulic working vehicle, and more particularly to a system for changing control patterns of hydraulic drive members for driving a boom assembly and a swivel deck of a backhoe.
2. Description of the Related Art
Such a system for changing control patterns of a boom assembly and a swivel deck of a backhoe is known from U.S. Pat. Nos. 4,398,861 and 4,736,647. The known system comprises hydraulic actuators for driving various parts of the boom assembly and the swivel deck, control valves for supplying and exhausting pressure oil to/from the hydraulic actuators, and a pair of right and left control levers operable fore and aft and right and left. The right and left control levers are mechanically connected to the control valves through push-pull wires and interlocking rods.
With fore and aft and right and left operation of the right and left control levers, the control valves are switched to operate the hydraulic actuators. More particularly, the righthand control lever, for example, is connected to the control valves such that its fore and aft operation moves a boom up and down, and right and left operation thereof causes a bucket to take a loading action. After the righthand lever is placed in a different interlocking relationship with the control valves, its fore and aft operation moves an arm of the boom assembly fore and aft, and right and left operation turns the swivel deck. In this way, control patterns of actuator drive by the control levers are changeable by relocating the push-pull wires and interlocking rods interconnecting the control levers and control valves.
When changing the control patterns as noted above, however, the push-pull wire and interlocking rods connecting one of the control levers to certain of the control valves must be dislodged once and placed in position to connect the other control lever to the same or different control valves. This operation is cumbersome, and it is almost impossible from the structural point of view to provide an increased number of control patterns available for selection.
A different type of control change system is known from Japanese Utility Model Publication No. 1989-24221 and Japanese Patent Publication No. 1989-60702. This system includes pilot pressure generators for supplying a pilot pressure in response to operation of control levers, control valves operable under the pilot pressure to control hydraulic actuators, and a direction changeover valve disposed between the pilot pressure generators and control valves. The direction changeover valve is operable to change control patterns.
However, this system also requires the number of valve positions corresponding to the number of control patterns made available. This results in a direction changeover valve having a large and complicated construction.